Influence of sepsis on higher daily dose of amikacin pharmacokinetics in critically ill patients

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1 European Review for Medical and Pharmacological Sciences Influence of sepsis on higher daily dose of amikacin pharmacokinetics in critically ill patients 2013; 17: L. MAHMOUDI 1, A.H. MOHAMMADPOUR 1, A. AHMADI 2, R. NIKNAM 3,4, M. MOJTAHEDZADEH 2,5 Department of Clinical Pharmacy, School of Pharmacy, Shiraz University of Medical Sciences, Shiraz, Iran. 1 Department of Pharmacodynamics and Toxicology, Pharmaceutical Research Center and School of Pharmacy, Mashhad University of Medical Sciences, Mashhad, Iran. 2 General Intensive Care Unit of Sina Teaching Hospital affiliated to School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. 3 Gastroenterohepatology Research Center, Shiraz University of Medical Sciences, Shiraz, Iran. 4 Gastroenterology and Hepatology Section, Fasa University of Medical Sciences, Fasa, Iran. 5 Department of Clinical Pharmacy, School of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran Abstract. BACKGROUND AND OBJECTIVES: Severe sepsis is a major problem as cause of high rates morbidity and mortality in intensive care units (ICU). Aminoglycosides are an important group of antimicrobials used for severe sepsis. However, aminoglycoside pharmacokinetics in ICU patients may be altered during sepsis, which can affect the drug concentrations. Therefore, this study was undertaken to examine the relationship between amikacin disposition kinetics after a 25 mg/kg loading dose and hemodynamic response to sepsis, as well as clinical parameters, in a population of critically ill patients. METHODS: In this work, 30 patients who were candidate to amikacin therapy following Gram negative sepsis were enrolled. The pharmacokinetic profile of amikacin by a non-compartmental model was calculated for each patient. RESULTS: Mean volume of distribution was 0.36 ± 0.07 L/kg and mean serum amikacin clearance was 3.88 ± 0.97 ml/min/kg. In the case of Vd, APACHE II score correlation was significant. In the case of amikacin clearance, two covariates including creatinine clearance and Sr Cr significant correlation was found. CONCLUSIONS: It appears necessary to use higher amikacin dosage ( 25 mg/kg) considering hemodynamic response of patients to sepsis. To achieve therapeutic drug concentration a close drug monitoring and a shift from the population mean toward a value more representative of the critically ill patient subpopulation is crucial. Key Words: Amikacin, Pharmacokinetics, Sepsis, Critically ill patients, Aminoglycosides. Introduction Treatment of infections in critically ill patients is a serious challenge for intensive care unit (ICU) physicians, because these infections are associated with high rates of mortality and morbidity 1. Up to 50% of all patients diagnosed with severe sepsis will die during their hospital admission 2. Selection of appropriate antibiotic is the mainstay of treatment, since an inappropriate initial choice may be responsible for higher therapeutic failure and higher mortality rates 3,4. Aminoglycosides are an important group of antimicrobial agents for severe sepsis and septic shock due to the desirable properties not exhibited by other classes of antibiotics 5. Amikacin is a highly potent aminoglycoside, extensively used in ICUs to treat patients with life-threatening Gram-negative infections, with an excellent therapeutic value 6. Amikacin has structural differences from the rest of aminoglycosides that makes it a more suitable option for the treatment of serious infections. Because of these structural differences, amikacin is not inactivated by common bacterial enzymes and a lower appearance of resistance has been observed with its use 7. Pharmacokinetic (PK)/pharmacodynamic (PD) relationship of amikacin, like other aminoglycosides, is concentration dependent and the ability to treat infections is strongly associated with a high peak serum concentration 8. Therefore, clinical estimates of amikacin dosage based on aver- Corresponding Author: Mojtaba Mojtahedzadeh, Pharm D, BCPS; mojtahed@sina.tums.ac.ir 285

2 L. Mahmoudi, A.H. Mohammadpour, A. Ahmadi, R. Niknam, M. Mojtahedzadeh Statistical Analysis Descriptive statistics were computed for all study variables. All data were entered to a database system. Correlation between groups was anage drug characteristics are inaccurate in critically ill patients, since aminoglycoside PK may be altered in ICU patients during sepsis, which can affect the drug concentrations 9. These PK alterations may lead to unpredictable and variable exposure to amikacin when administered to these patients at conventional doses. Thus, optimizing amikacin dosing regimen to enhance antibacterial activity and therapeutic response might need higher doses of aminoglycosides 10. Although the PK disposition of aminoglycosides has been studied before 7,11,12, limited PK data are available on higher dose amikacin in critically ill septic patients 10,13. Moreover, the interpretation of this data is complicated since usually patients admitted to ICUs are not a homogenous population, due to diverse underlying diseases and severities of the illness 14. Therefore, this study was undertaken in a population of critically ill patients to examine the relationship between the amikacin disposition kinetics after a 25 mg/kg loading dose and the hemodynamic response to sepsis, as well as clinical parameters. Methods Study Population In this study, 30 patients with a diagnosis of severe sepsis 15 hospitalized in a general ICU between March 2010 and March 2011 were recruited. The legally authorized representative of each eligible patient was informed about the objectives of study and a written consent was obtained. This study was approved by our Institutional Investigational Review Board for human and animal studies. Patients, who were candidate to receive amikacin following Gram-negative sepsis, were enrolled. Patients were excluded according to the following criteria: age under 18 or over 65 years, pregnancy, extensive burns ( 20% of body surface area), ascites, inadequate renal function (creatinine clearance < 60 ml/min), known cochleovestibular abnormalities, myasthenia gravis, and allergy to aminoglycosides, meningitis, endocarditis, immunodeficiency, and treatment with aminoglycosides in the previous 2 weeks. During the empiric antibiotic therapy, amikacin with a loading dose of 25 mg/kg (intravenously over 60 minutes) was co-administered with a β-lactam antibiotic or carbapenem. Drug Assays In each case, 5 ml of blood was collected by infusion set following establishment of good laboratory practices at 0, 0.25, 0.5, 1.5, 4.0, 8.0, 12.0, 18 and 24 hours after the start of i.v. infusion in a 5-ml plain tube (without anticoagulant). When a clot had completely formed (15 to 30 min), the sample was centrifuged at 4 C, and the serum was stored at 80 C until analysis. Serum concentrations of amikacin were quantified at the end of the study by means of fluorescence polarization immunoassay with the TDx analyzer (Abbott GmbH, Wiesbaden, Germany). According to the manufacturer, the limit of quantitation for this assay was 0.6 mg/l and the analytical precision was 3.18%, 2.62%, and 2.50% for low (5 mg/l), medium (15 mg/l), and high (30 mg/l) concentrations, respectively. Data Collection Whenever a blood sample was taken, all relevant demographic data (e.g. age, gender, total body weight (TBW), disease severity as characterized by the Acute Physiology and Chronic Health Evaluation (APACHE) II score 16, central vein pressure (CVP) were recorded. In addition several laboratory tests including complete blood count (CBC), serum creatinine (Sr Cr), blood urea nitrogen (BUN), liver function tests, and electrolytes were performed at inclusion and at 24 hours. Creatinine clearance (CrCl) was estimated with the Cockcroft and Gault equation by using TBW 17. Pharmacokinetic The pharmacokinetic profile of amikacin was analyzed by a non-compartmental model 18 and following pharmacokinetic variables were calculated for each patient. Total volume of distribution (Vd), total body clearance (Cl), elimination half-life (t 1/2 ), area under the curve (AUC) during the 24 hours, Peak (amikacin level measured 30 min after the completion of infusion) and trough (concentration 24 h after the start of infusion). Peak > 64 µg/ml was considered optimal concentration by expectance a mean Vd of 0.4 L/kg and the threshold of toxicity was determined by a trough > 5 µg/ml 10,

3 Influence of sepsis on higher daily dose of amikacin pharmacokinetics in critically ill patients alyzed by using the Pearson correlation test and comparison between two groups was analyzed by the use of one sample t-test. (SPSS software for windows, version 11.5; SPSS Inc., Chicago, IL, USA). p value less than 0.05 was considered significant. Results Characteristics of the Study Population The study population consisted of 30 critically ill patients with the diagnosis of severe sepsis in which amikacin treatment was indicated. Demographic and laboratory tests of the patients are shown in Table I. Pharmacokinetic Data The median amikacin dose was 1500 mg ( ) which was administered intravenously over 60 minutes. Main PK parameters for amikacin are presented in Table II. The mean value (± standard deviation) of amikacin plasma concentration-time profiles of patients are shown in Figure 1. The current value of Vd 19 and Cl 20 of amikacin observed in critically patients were 0.36 ± 0.07 L/kg and 3.88 ± 0.94 ml/ min/kg respectively, in comparison with a Vd of 0.25 L/kg and Cl of 1.37 ml/min/kg in non-critically ill patients, by using one sample t-test (p < 0.05). Therapeutic Drug Monitoring of Amikacin The peak serum concentrations were > 64 µg/ml in 75% of patients (Figure 2). No patient had trough concentration > 5 µg/ml (Figure 3). Table I. Characteristics of the study population. Characteristics Value Demographic data Age (years) ± 14.2 Total Body Weight (kg) ± 9.57 Male/ Female (%) 70/30 APACHE II score ± 5.54 Mean arterial pressure (mmhg) 78 ± 11 Central venous pressure (mmhg) 12 ± 4 ph 7.41 ± 0.06 PaCO 2 (mmhg) ± 7.18 HCO 3 (mmol/l) ± 3.82 Laboratory tests AST(U/L) ± ALT(U/L) ± Sr Cr(mg/dl) 0.82 ± 0.18 BUN(mg/dl) ± Cr Cl (ml/min/1.73 m 2 ) 69 ± 7.53 WBC (/µl) ± Hb (g/dl) 9.93 ± 1.48 Platelets (/µl) ± Na (meq/l) ± 4.32 K(meq/L) 3.40 ±.52 Albumin(mg/dl) 2.97 ± 0.47 Urine output (ml/24 h) ± Data are expressed as percentage or mean ± SD, Sr Cr: serum ceratinine, BUN: Blood Urea Nitrogen, APACHE: Acute Physiology and Chronic Health Evaluation, Cr Cl: creatinine clearance, Alb: serum albumin concentration. Relationship Between Amikacin Pharmacokinetic Values and Patients Variables Relationship between amikacin Vd and Cl and patient s age, TBW, APACHE II score, Cr Cl and Sr Cr, as determined by Pearson correlation test, is shown in Table III. The APACHE II score s correlation was significant with amikacin Vd (p < 0.05). In the case of amikacin Cl, two covari- Table II. Pharmacokinetic parameters of the studied population. Parameter Minimum Maximum Mean ± SD CV (%) AUC (mg hr/ml) ± AUMC ± MRT (hr) ± t 1/2 (hr) ± Cl (ml/min/kg) ± Vd (L/kg) ± Peak (µg/ml) ± Trough (µg/ml) ± AUC: area under the serum level-time curve, AUMC: area under the moment versus time curve, MRT: mean residence time, t 1/2 : elimination half-time, Cl: total body clearance, Vd: volume of distribution, CV: coefficient of variation 287

4 L. Mahmoudi, A.H. Mohammadpour, A. Ahmadi, R. Niknam, M. Mojtahedzadeh Concentrations mg/l Patients (%) Time (h) Figure 1. Mean concentration-time (± SD) profile of amikacin. Serum trough concentrations (µg/ml) Figure 3. Distribution of trough concentrations. Patients (%). Patients (%) Peak serum trough concentrations (µg/ml) Figure 2. Distribution of peak concentrations. Black bars, peak > 64 µg/ml; gray bars, peak < 64 µg/ml. Patients (%). ates including Cr Cl and Sr Cr significant correlation was found (p < 0.05). No significant correlations between other variables and amikacin Cl and Vd were found. Discussion As far as we know, limited studies have described higher dose amikacin in ICU septic patients 10,13. Our study shows the pharmacokinetic values of a 25 mg/kg loading dose of amikacin and the influence of patients variables on this higher dose in a population of severe septic patients. Table III. Relationship between amikacin PK values and patients variables. Variable Vd Cl t 1/2 C max Age 0.36 (0.122) 0.12 (0.424) 0.13 (0.596) (0.026) TBW (0.568) 0.40 (0.079) (0.004) (0.370) APACHE II 0.45 (0.044) 0.14 (0.567) 0.24 (0.309) (0.244) Cr Cl (0.279) 0.49 (0.027) (0.001) 0.12 (0.627) Sr Cr (0.932) (0.025) 0.45 (0.047) 0.63 (0.003) Data are expressed as Pearson Correlation: r2 (p value), Vd: volume of distribution, Cl: total body clearance, t 1/2 : elimination half-time, C max : peak concentration, TBW: total body weight, BUN: blood urea nitrogen, APACHE: Acute Physiology and Chronic Health Evaluation, Cr Cl: creatinine clearance, Sr Cr: serum creatinine. 288

5 Influence of sepsis on higher daily dose of amikacin pharmacokinetics in critically ill patients The present study demonstrated that a profound change in amikacin disposition kinetics happens in response to sepsis in critically ill patients. Therefore, a higher dose schedule of amikacin might be needed to achieve optimal serum concentrations. Critically ill patients normally are not a homogenous population, because there are varieties in the patient presentation, differences in underlying disease, levels of disease severity and organ function 14. Most dosing regimens applied in ICU patients are founded on clinical trials involving non-icu patients or healthy volunteers, based on the assumption that PK variables are similar between these two groups 21,22. However, several pathophysiological changes occurring in ICU patients with sepsis can affect drugs PK behavior 10. Hyperdynamic conditions, including an increased basal metabolic energy expenditure (BEE), cardiac output and oxygen consumption, frequently occur in the early phase of sepsis in response of body to infective insult 12. Moreover, dynamic changes in volume status and end organ dysfunction can lead to altered PK properties of many drugs 22. Various factors concerning the disease are correlated to aminoglycoside pharmacokinetics in the critically ill patients 23 who often display an increased Vd for aminoglycosides and this leads to a decreased peak serum concentration 6. In our investigation the mean Vd was 0.36 ± 0.07, significantly higher than the Vd of 0.25 L/kg in non-critically ill patients 19. This finding was consistent with most reported values in septic patients, confirmed that sepsis induced Vd change under similar conditions. Changes of Vd in critically ill patients were reported by Triginer et al 24. In this study, in a group of critically ill patients, Vd in the first phase of the research was 0.43 ± 0.12 L/kg. Lugo et al 12 evaluated amikacin disposition in 30 critically ill patients and reported that mean Vd of amikacin was 0.47 L/kg, a value significantly increased in comparison with healthy volunteers. Taccone et al 10 and Marik et al 25, using amikacin in ICU patients with severe Gram-negative infections, referred that the mean Vd of amikacin was 0.37 L/kg and 0.41 L/kg respectively. Different factors are responsible of changes in Vd in critically ill patients. Endotoxins from bacteria can lead to the production of numerous mediators affecting vascular endothelium and increased permeability 26. Edematous status may cause clinical failure of antimicrobial therapy via increasing Vd and lowering antimicrobial concentrations 27. Furthermore, capillary permeability, fluid therapy and total parenteral nutrition (TPN) may contribute in critically ill patients to expand the extracellular fluid and Vd and, as a result, a greater dilution of the aminoglycosides 28. Increased sickness severity, as measured by the APACHE II score, has been shown to be related to higher Vd for aminoglycosides 29. According to our findings, amikacin Vd was affected by patients APACHE II score. Data reported by Marik et al 29 and Lugo et al 30 as well as our data, support an association between sepsis severity, as estimated by the APACHE II score, and the expansion of the aminoglycoside Vd. According to our results, the amikacin Cl in septic patients was significantly higher than amikacin Cl in non-icu patients likewise to Tang et al 23 who found that the clearance of gentamicin was 1.5-fold higher in hyperdynamic septic patients. In the critically septic patients, physiopathologic changes determined by sepsis and associated stress, hemodynamic status and medical interventions determine a wide variability in aminoglycoside Cl. The body response to infection and injury lead to the increase of BEE, oxygen consumption, and higher cardiac output, blood flow and organ perfusion. In severe sepsis cardiac index is normal or even increased compared with that in the absence of organ dysfunction; renal artery blood flow is also increased resulting in the enhanced delivery and excretion of hydrophilic agents like aminoglycosides 31. Amikacin is primarily eliminated by glomerular filtration and, thus, modifications in renal function should directly affect the drug s clearance 32. In our results, there was a significant correlation between amikacin Cl and Cr Cl that was similar to other researches 12,33. The loading dose is directly proportional to the drug Vd 34, and an optimal regimen for aminoglycoside therapy would assure an adequate initial drug level, thus minimizing the opportunity for adaptive resistance 35. According to Jackson et al 35, the primary bactericidal phase is rapid and drug concentration dependent. During this initial phase, the killing rate is directly related to the initial drug concentration. 75% of our patients achieved the target peak concentration of > 64 µg/ml by using a 25 mg/kg dose of amikacin. But still 25% of them had a peak concentration below the target level and it shows the necessity of using even higher doses in these patients. 289

6 L. Mahmoudi, A.H. Mohammadpour, A. Ahmadi, R. Niknam, M. Mojtahedzadeh Nephrotoxicity of aminoglycosides has been associated with elevated trough concentrations 36. In our work, trough concentrations were well below 5 µg/ml for all patients. Optimizing aminoglycoside concentrations according to patient pharmacokinetic model is crucial to enhance efficacy while minimizing complications due to toxicity 5. Therefore, based on our findings, by using amikacin dose of 25 mg/kg, although most of patients reached optimal peak level, we couldn t achieve target peak concentration in a portion of our patients. Since amikacin is known to be a concentration-dependent antimicrobial drug, it shows that by individualizing dosage regimen, and probably using a dose of 25 mg/kg, we may enhance efficacy by optimizing peak concentration. Meanwhile, because of low trough levels in this regimen it could help reducing the risk of amikacin nephrotoxicity. Two major limitations exist in our investigation. First, a control group without sepsis was not included, because administration of a higher dose of a potentially toxic antibiotic to nonseptic patients was not ethical. Second, our study was limited to first 24 hours of amikacin administration. However, resolution of sepsis resulted in decline of Vd and Cl. In conclusion, our study have showed the importance of factors associated with physiologic response of patients due to sepsis, and their clinically relevant influence on the variability of pharmacokinetic profile of amikacin. Subtherapeutic levels of amikacin possibly due to the increase in Vd and Cl, represents a serious challenge for achieving target concentrations rapidly and efficiently, and thus control of the septic process. Therefore, it appears necessary to adjust amikacin dosage considering hemodynamic response of patients and a shift from the population mean toward a value more representative of the critically ill patient subpopulation. To maximize amikacin antibacterial effect, a close drug monitoring and dosing of amikacin at high doses ( 25 mg/kg) is probably required. This will help in defining more precisely optimal peak concentrations of amikacin that provide the greatest effectiveness with least toxicity. References 1) BRUN-BULSSON C. ICU-acquired infections and sepsis: more of a deadly duo. Intensive Care Med 2008; 34: ) ZENI F, F REEMAN B, NATANSON C. Anti-inflammatory therapies to treat sepsis and septic shock: a reassessment. Crit Care Med 1997; 25: ) KUMAR A, ROBERTS D, WOOD KE, LIGHT B, PARRILLO JE, SHARMA S, SUPPES R, FEINSTEIN D, ZANOTTI S, TAIBERG L, GURKA D, KUMAR A, CHEANG M. Duration of hypotension before initiation of effective antimicrobial therapy is the critical determinant of survival in human septic shock. Crit Care Med 2006; 34: ) LEIBOVICI L, SHRAGA I, DRUCKER M, KONIGSBERGER H, SAMRA Z, PITLIK SD. The benefit of appropriate empirical antibiotic treatment in patients with bloodstream infection. J Intern Med 1998; 244: ) FAUVELLE F, PERRIN P, BELFAYOL L, BOUKARI M, CHERRIER P, B OSIO AM, TOD M, COULAUD JM, PETITJEAN O. Fever and associated changes in glomerular filtration rate erase anticipated diurnal variations in aminoglycoside pharmacokinetics. Antimicrob Agent Chemother 1994; 38: ) VANHAEVERBEEK M. Pharmacokinetics of once-daily amikacin in elderly patients. Antimicrob Agent Chemother 1993; 31: ) LORTHOLARY O, TOD M, COHEN Y, P ETITJEAN O. Aminoglycosides. Med Clin North Am 1995; 79: ) OLSEN KM, RUDIS MI, REBUCK JA, HARA J, GELMONT D, MEHDIAN R, NELSON C, RUPP ME. Effect of once-daily dosing vs. multiple daily dosing of tobramycin on enzyme markers of nephrotoxicity. Crit Care Med 2004; 32: ) BUIJK SE, MOUTON JW, GYSSENS IC, VERBRUGH HA, BRUINING HA. Experience with a once-daily dosing program of aminoglycosides in critically ill patients. Intens Care Med 2002; 28: ) TACCONE FS, LATERRE PF, SPAPEN H, DUGERNIER T, DE- LATTRE I, LAYEUX B, DE BACKER D, WITTEBOLE X, WALLEMACQ P, V INCENT JL, JACOBS F. Revisiting the loading dose of amikacin for patients with severe sepsis and septic shock. Crit Care 2010; 14: R53: ) BRESSOLLE F, GOUBY A, MARTINEZ JM, JOUBERT P, SAISSI G, GUILLAUD R, GOMENI R. Population pharmacokinetics of amikacin in critically ill patients. Antimicrob Agents Chemother 1996; 40: ) LUGO G, CASTANEDA-HERNANDEZ G. Relation between hemodynamic and vital support measures and pharmacokinetic variability of amikacin in critically ill patients with sepsis. Crit Care Med 1997; 25: ) DELATTRE IK, MUSUAMBA FT, NYBERG J, TACCONE FS, LATERRE PF, VERBEECK RK, JACOBS F, WALLEMACQ PE. Population pharmacokinetic modeling and optimal sampling strategy for Bayesian estimation of amikacin exposure in critically ill septic patients. Ther Drug Monit 2010; 32: ) YAMAMURA S. Clinical application of artificial neural network (ANN) modeling to predict pharmacokinetic parameters of severely ill patients. Adv Drug Deliv Rev 2003; 55:

7 Influence of sepsis on higher daily dose of amikacin pharmacokinetics in critically ill patients 15) BONE RC, FISHER CJ JR, CLEMMER TP, SLOTMAN GJ, METZ CA, BALK RA. Sepsis syndrome: A valid clinical entity. Crit Care Med 1989; 17: ) KNAUS WA, DRAPER EA, WAGNER DP, ZIMMERMAN JE. APACHE II: A severity of disease classification system. Crit Care Med 1985; 13: ) COCKCROFT DW, GAULT MH. Prediction of creatinine clearance from serum creatinine. Nephron 1976; 16: ) NORWICH KH. Non compartmental models of whole-body clearance of tracers: A review. Ann Biomed Eng 1997; 25: ) TRIGINER C, IZQUIERDO I, FERNÁNDEZ R, TORRENT J, BENITO S, NET A, JANE F. Changes in gentamicin pharmacokinetic profile induced by mechanical ventilation. Eur J Clin Pharmacol 1991; 40: ) BAUER LA, EDWARDS WA, DELLINGER EP, SIMONOWITZ DA. Influence of weight on aminoglycoside pharmacokinetics in normal weight and morbidly obese patients. Eur J Clin Pharmacol 1983; 24: ) MCKINDLEY DS, HANES S, BOUCHER BA. Hepatic drug metabolism in critical illness. Pharmacotherapy 1998; 18: ) POWER BM, FORBES AM, VAN HEERDEN PV, ILETT K. Pharmacokinetics of drugs used in critically ill adults. Clin Pharmacokinet 1998; 34: ) TANG GJ, TANG JJ, LIN BS, KONG CW, LEE TY. Factors affecting gentamicin pharmacokinetics in septic patients. Acta Anaesthesiol Scand 1999; 43: ) TRIGINER C, IZQUIERDO I, FERNANDEZ R, RELLO J, TOR- RENT J, BENITO S, NET A. Gentamicin volume of distribution in critically ill septic patients. Intensive Care Med 1990; 16: ) MARIK PE, HAVLIK I, MONTEAGUDO FS, LIPMAN J. The pharmacokinetic of amikacin in critically ill adult and paediatric patients: comparison of once-versus twice-daily dosing regimens. J Antimicrob Chemother 1991; 27: ) BONE RC. The pathogenesis of sepsis. Ann Intern Med 1991; 115: ) HANSEN M, CHRISTRUP LL, JARLØV JO, KAMPMANN JP, BONDE J. Gentamicin dosing in critically ill patients. Acta Anaesthesiol Scand 2001; 45: ) TORMO C, ABAD FJ, RONCHERA-OMS C.L, PARRA V, JIMENEZ N.V. Critically-ill patients receiving total parentral nutrition show altered amikacin pharmacokinetics. Clin Nutr 1995; 14: ) MARIK PE. Aminoglycoside volume of distribution and illness severity in critically ill septic patients. Anaesth Intens Care 1993; 21: ) LUGO-GOYTIA G, CASTANEDA-HERNANDEZ G. Bayesian approach to control of amikacin serum concentrations in critically ill patients with sepsis. Ann Pharmacother 2000; 34: ) PEA F, V IALE P. The antimicrobial therapy puzzle: could pharmacokinetics-pharmacodynamics relationships be helpful in addressing the issue of appropriate pneumonia treatment in critically ill patients? Clin Infect Dis 2006; 42: ) ZASKE DE, CIPOLLE RJ, ROTSCHAFER JC, KOHLS PR, STRATE RG. Individualizing amikacin regimens; accurate method to achieve therapeutic concentrations. Ther Drug Monit 1991; 13: ) FIRSOV AA, NASONOV VN. Correlation between aminoglycoside pharmacokinetic parameters and patient s factors: From statement to implication for individual dosage design. Eur J Drug Metab Pharmacokinet 1991; 3: ) WADA DR, DROVER DR, LEMMENS HJ. Determination of the distribution volume that can be used to calculate the intravenous loading dose. Clin Pharmacokinet 1998; 35: ) JACKSON GG, LOLANS VT, DAIKOS GL. The inductive role of ionic binding in the bactericidal and postexposure effects of aminoglycoside antibiotics with implications for dosing. J Infect Dis 1990; 162: ) FISMAN DN, KAYE KM. Once-daily dosing of aminoglycoside antibiotics. Infect Dis Clin North Am 2000; 14: